feat(store): draft states and discards are journaled, posted_seq is kept (P4)

Accounting reason: a draft that waits on its receipt, and one thrown
away, are facts about the books' working papers that must survive a
restart and replay identically, or a discarded draft would come back and
hold its period open. entry.draft_state_set and entry.draft_discarded are
drafts_v2's (an older version stops at the declaration instead of
misreading them); no figure moves. Schema 19 (taken at merge as the next
after the current last): entries.posted_seq, and a draft_states table
(drafts are not rows of entries, so the plan's entries.draft_state column
has nowhere to live). posted_seq is the journal line that posted each
entry (entry.posted, entry.reversed, the line that closed a period or
posted a schedule month), identical live and on replay.

8.0.6: a hand-written line of each new shape in
books_written_under_older_rules_still_open; fixture old-books/p4_before
written by 25b52580 (hand entries posted late, a reversal, a document, a
schedule month, a recurring draft, open drafts, a close) opens to the
same figures (serialized entries omit a building state, so no golden
moved); drafts_v2_is_its_own_feature; the fuzzer and GUI journal know the
types.

Tests: draft_states_replay_and_project, drafts_v2_waits_for_the_upgrade,
posted_seq_is_the_posting_lines_seq, an_older_version_stops_at_the_draft_events.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

crates/ledger-core/src/journal.rs

@@ -63,6 +63,12 @@ impl DraftState {
         }
     }
 
+    /// Whether this is the default, building (a serialized entry omits it,
+    /// so every entry an earlier version wrote reads and writes as it did).
+    pub fn is_building(&self) -> bool {
+        matches!(self, DraftState::Building)
+    }
+
     /// The note an awaiting state carries; empty for building.
     pub fn note(&self) -> &str {
         match self {
@@ -110,7 +116,7 @@ pub struct JournalEntry {
     reversal_of: Option<EntryId>,
     /// Where the draft stands (`entry.draft_state_set`); meaningful while
     /// `status` is `Draft`.
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "DraftState::is_building")]
     draft_state: DraftState,
 }